The Institute of Electrical and Electronic Engineering (IEEE) 8002.3af standard 802.3af-2003 allows a PD (Powered Device) to report power requirements and request to be powered at that level through the classification protocol. Future standard 802.3at will provide for much higher power to be delivered as well as much more granularity on power required requests. Standard integrated 802.3af-compliant PD devices are typically hard-coded to report a fixed power requirement through classification. It is expected that integrated 802.3at devices will work under the same premise of assuming a fixed power classification level.
In a modular PD system, a fixed classification reporting is not appropriate. Required power can vary greatly between sub-modules (e.g., radios for access point systems). However, a dynamic classification request based on module population is complicated by several factors. For example, the 802.3af/at classification hardware must determine module power requirements—classification circuitry is on the Ethernet line side and modules are located on isolated side of primary power supply. In addition, 802.3af/at classification hardware must present appropriate classification signature based upon module population. Another problem is that the 802.3af/at classification hardware communicates to an isolated side main system controller what level of power is available. For example whether the Power Sourcing Equipment “PSE” switch is 802.3af or 802.3at compliant, or what level of 802.3at power the PSE can supply. When the host controller knows the power available from the PSE, the host controller can enable/disable appropriate modules, or operate the modules in a mode to insure that PSE power capabilities are not exceeded.